LED-based emergency lighting equipment and methodology

ABSTRACT

Systems and methods provide LED-based emergency lighting utilizing AC-DC switch mode power conversion technology, NiMH battery technology, and emergency lighting lamps that use high power white LEDs as the emergency lighting source. A low voltage microprocessor based circuit design reduces the battery input voltage for the unit to a nominal level of 2.4 VDC. The microprocessor executes a pulse charging algorithm to lower battery maintenance mode power consumption levels and extend the useful life of the battery. Brownout detection technology does not require the determination of the AC input voltage level or transmission of the brownout detection signal to the secondary side of the circuit. A rechargeable battery is charged by a charge current selectively set to a bulk charge value, a trickle charge high value, or a trickle charge low value based on sampling of the voltage of the rechargeable battery and the charge current.

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to U.S. Pat. No. 8,258,705 issued Sep. 4, 2012,and pending U.S. patent application Ser. No. 13/220,002 filed Aug. 29,2011, the disclosures of which (including all attachments filedtherewith) are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is in the field of LED-based emergency lighting.Generally, LED-based emergency lighting systems, equipment andmethodologies according to exemplary embodiments of the presentinvention utilize AC-DC switch mode power conversion technology, NiMHbattery technology, and emergency lighting lamps that use high powerwhite LEDs as the emergency lighting source.

2. Discussion of the Background of the Invention

For many years commercial two lamp emergency lighting unit designs haveprimarily been based around three key design elements to achieve coststhat were acceptable in the marketplace while delivering reasonablelevels of performance lighting the path of egress in emergencysituations. These three design elements in most emergency lighting unitsinclude (1) Valve Regulated Lead Acid (VRLA) batteries, (2) incandescentDC lamps, and (3) 60 Hz transformer based voltage controlled chargers.These elements combine to provide typical entry level emergency lightingunits that are widely used and stocked by electrical distributors tomeet the basic emergency lighting needs.

The choice of these traditional design elements for emergency lightingunits has typically been dictated by a narrow range of DC light sourcesthat could light the path of egress from battery power after a loss ofAC power to the unit. Incandescent lamps for many years were the onlysources that could offer the amount of light required for theapplication at a reasonable price point. Traditional entry levelemergency lighting units typically use two 6V 5.4 W incandescent lamps.Because these incandescent light sources have relatively low efficacylevels, the batteries required to power them for the 90 minute run timesmandated by NFPA 101 life safety codes were heavy and bulky VRLA types.The typical battery size for most entry level emergency lighting unitsis the 6V 4 Ah VRLA battery. This battery size is convenient because itmatches the voltage rating of the lamp and can deliver code mandated runtimes with two 6V 5.4 W incandescent lamp sources. The 6V system is alsowidely used because it allows the use of a simple relay to transfer thebattery power to the lamps during emergency conditions when AC power islost to a building.

Because the battery sizes of the traditional emergency lighting unit arerelatively large the chargers used to keep the batteries in a fullycharged state also have to be sized to charge these larger batteries.Typical entry level emergency lighting units employ a 60 Hz magneticstep down transformer and a voltage controlled charging regulator toprovide a well regulated dc voltage source needed to maintain thebatteries in a fully charged state. Thus, one of the problems fortraditional emergency lighting units is the need for relatively largeand heavy batteries and battery chargers.

With recent advances in white high power LED lighting technology LEDshave reached luminous output levels where they can provide light levelsequivalent to the traditional lamps technologies at much lower powerlevels. Because of the higher level of efficacy for the LED source, muchlower AC input power requirements can be achieved for general purposelighting fixtures. This significant energy usage reduction for the LEDsource has driven rapid adoption of the LED as a light source in generalpurpose, normally on, lighting fixtures in recent years. While theadvances in LED lighting technology have made the LED a viablereplacement lighting source in general purpose lighting fixtures thecosts associated with the LED light source are significantly higher thanmore established traditional light sources. The disadvantageous highcosts are likewise associated with conventional emergency lighting unitsattempting to use LED light sources.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention address at least theabove problems and/or disadvantages and provide at least the advantagesdescribed below.

An exemplary embodiment of the present invention provides LED-basedemergency lighting system comprising AC-DC switch mode power converter,a rechargeable NiMH battery, and an emergency lighting lamp includinghigh power white LED lighting source. In an exemplary implementation, alow voltage microprocessor based circuit design reduces the batteryinput voltage for the unit to a nominal level of 2.4 VDC.

According to an exemplary implementation of an embodiment of the presentinvention, the system comprises a microprocessor executing a pulsecharging algorithm to lower battery maintenance mode power consumptionlevels and extend the useful life of the battery.

In another exemplary implementation of an embodiment of the presentinvention, a system utilizes brownout detection technology that does notrequire the determination of the AC input voltage level or transmissionof the brownout detection signal to the secondary side of the circuit.

Yet another exemplary embodiment of the present invention provides amethod for charging a rechargeable battery by a charge current includingsampling a voltage of the rechargeable battery and the charge current,and then setting the charge current based on the results of the samplingto a bulk charge value, a trickle charge high value, or a trickle chargelow value.

Yet another exemplary embodiment of the present invention provides amethod for charging a rechargeable battery by a charge current includingsampling a voltage of the rechargeable battery and the charge current,calculating a delta V/delta t value of the rechargeable battery voltage.Then the charge current is set based on the results of the sampling andthe delta V/delta t calculation to a bulk charge value, a trickle chargehigh value, or a trickle charge low value.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a circuit diagram illustrating associated circuitry forbrown-out function and AC-DC switch mode controller according toexemplary embodiments of the present invention.

FIG. 2 is a circuit diagram illustrating an exemplary LED unit that canbe employed in emergency lighting according to exemplary embodiments ofthe present invention.

FIG. 3 is a block I/O diagram illustrating an exemplary implementationof a microprocessor for controlling emergency lighting according toexemplary embodiments of the present invention.

FIGS. 4A-4C and 5A-5F are circuit diagrams illustrating additionalcomponents of LED-based emergency lighting according to exemplaryembodiments of the present invention.

FIG. 6 is a state machine diagram of an exemplary unit system substratemachine for LED based emergency lighting equipment according to anexemplary embodiment of the present invention.

FIG. 7 is a state machine diagram of another exemplary unit systemsubstrate machine for LED based emergency lighting equipment accordingto another exemplary embodiment of the present invention.

FIG. 8 is a state machine diagram of an exemplary battery chargesubstrate machine for LED based emergency lighting equipment accordingto an exemplary embodiment of the present invention

FIG. 9 is a state machine diagram of an exemplary LED driver substratemachine for LED based emergency lighting equipment according to anexemplary embodiment of the present invention.

FIG. 10 is a process flow diagram illustrating an exemplaryinitialization state for LED based emergency lighting equipmentaccording to an exemplary embodiment of the present invention.

FIGS. 11A and 11B are process flow diagram illustrating an exemplarytest/load-learn state for LED based emergency lighting equipmentaccording to an exemplary embodiment of the present invention.

FIG. 12 is a process flow diagram illustrating an exemplary emergencystate for LED based emergency lighting equipment according to anexemplary embodiment of the present invention.

FIG. 13 is a process flow diagram illustrating another exemplaryemergency state for LED based emergency lighting equipment according toanother exemplary embodiment of the present invention.

FIG. 14 is a state diagram illustrating an exemplary test state for LEDbased emergency lighting equipment according to an exemplary embodimentof the present invention.

FIG. 15 is a state diagram illustrating an exemplary load-learn statefor LED based emergency lighting equipment according to an exemplaryembodiment of the present invention.

FIG. 16 is a plot illustrating an exemplary battery charge cycleaccording to exemplary embodiments of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views,embodiments of the present invention are shown in schematic detail.

The matters defined in the description such as a detailed constructionand elements are nothing but the ones provided to assist in acomprehensive understanding of the invention. Accordingly, those ofordinary skill in the art will recognize that various changes andmodifications of the embodiments described herein can be made withoutdeparting from the scope and spirit of the invention. Also, well-knownfunctions or constructions are omitted for clarity and conciseness.Certain exemplary embodiments of the present invention are describedbelow in the context of commercial application. Such exemplaryimplementations are not intended to limit the scope of the presentinvention, which is defined in the appended claims.

Certain terms of art that may be used in the description have commonlyaccepted definitions as noted below:

Because the lamps in emergency lighting units are normally off themajority of the time when AC power is present in the building thereduced energy usage of the LED lamp source does not have a significantimpact on energy usage of the emergency lighting unit to the extent thatit has in normally on general purpose lighting fixtures. Although theLED does not offer a significant enough reduction in energy usage tojustify its use based on an energy usage pay back analysis it does havesignificant advantages over the traditional incandescent light sourcesthat make it a desirable and superior for emergency lighting unitapplications.

The four primary reasons to adopt the LED as a replacement light sourceover the traditional LED light source are higher luminous efficacy,increased life/reliability, reduction in required battery size and areduction in the overall size of the emergency lighting unit.

While it can be shown that there are many benefits to an LED basedemergency lighting unit, without a payback justification market adoptionof the technology would most likely be very slow. To accelerate themarket adoption of the LED technology in emergency lighting units therewere several problems that had to be overcome to offer an emergencylighting unit at a price point that was equivalent to the traditionalemergency lighting unit solutions.

In order to offset the higher cost of the LED light source, exemplaryembodiments of the present invention provide several design tradeoffsbased around the higher efficacy of the LED light source.

An exemplary embodiment of the present invention provides a systemcomprising two 1 W high brightness white LEDs. Based on estimated valuesof the light output from the LEDs and factoring in efficiency losses forLED heat sinks, optics and the driver it was determined that the LEDlight sources chosen would be capable of delivering 40% more coveragearea along the path of egress while using a battery that had 85% lesscapacity than the 6V 4 Ah battery used in incandescent based emergencylighting units. This huge reduction in required battery capacity todeliver more light output was a factor in realizing the four primarybenefits of the LED source while enabling several features that aredesirable for users of emergency lighting units.

According to exemplary embodiments of the present invention, because ofthe reduced battery capacity requirements it was viable to adopt aNickel Metal Hydride (NiMH) battery over the traditional VRLA batteryand still achieve a cost savings that would partially offset theincreased cost of the LED source. In an exemplary implementation, thisbattery eliminates the use of the Restricted and Hazardous Substance(RoHS) “lead” in the product with a much more environmentally friendlypower source. The use of the NiMH battery can also enable the use of abattery that has a typical life of 5 years vs. the 3 year life typicallyoffered by VRLA type batteries. The reduced battery size can also enablethe use of a much smaller housing size for the emergency lighting unit.According to an exemplary implementation, a reduction in size of thehousing can enable the use of less plastic to create the emergencylighting unit again offsetting the cost of the higher cost LED source.Because the LED source could offer 40% more light coverage area, anadditional benefit of less units to buy and install can be realized forthe customer Finally because the LED source is used, the unit can offera light source that can last the lifetime of the unit instead of atypical 50 hour lamp life that is traditionally realized for DCincandescent lamps. This increased lamp life can further reducemaintenance required to keep the unit operating properly over itslifetime.

According to further exemplary embodiments of the present invention,because an LED light source and a lower capacity NiMH battery werechosen as the key design elements for this invention a change in thebattery charging and transfer circuitry was required. These changes tookadvantage of the benefits that the LED source offered to provide severalnovel benefits to the customer while solving problems associated withthe new circuit topology.

According to an exemplary implementation of the present invention, areduction in battery size can be achieved by a reduction in the requirednominal battery voltage and nominal battery capacity.

Conventional emergency lighting units as described above have typicallyrelied on a VRLA 6V battery as the emergency power source for 6Vincandescent lamps. It was a relatively easy process to design a circuitthat detected the loss of AC power to the emergency lighting unit andutilized a relay to connect the battery to the lamps when a loss of ACpower was detected. Because the battery voltage needed to be reduced to2.4V for this design to maintain the required reduction in cost a simpledesign of this nature was not possible for this unit. Furthercomplicating the design process was the fact that the 2.4V batteryvoltage would need to be boosted to a level sufficient to power the twoLEDs in series while regulating the current through the series connectedLED string. While many commercially available DC-DC switch-modecontroller integrated circuits (ICs) exist in the market very few aredesigned to work from a 2.4V nominal voltage source. Of those fewcommercially available sources for DC-DC controller ICs that werecapable of meeting the design requirements none could do so at a costthat would be practical to achieve price parody with the conventionalemergency lighting solution.

To address the above-noted problem at a price point viable for thedesign, an exemplary embodiment of the present invention implements ageneral purpose microcontroller. While the microcontroller can be moreexpensive than many dedicated DC-DC controller ICs, it offered manyadvantages. Because of the general purpose nature of the microcontrollerthe DC-DC controller requirements could be integrated into one devicethat would not only fulfill those requirements but would also offer thefunctionality required to provide AC line loss detection, chargercontrol and the ability to monitor the battery, lamps and LED driverassociated with the design. Because microcontrollers are often used inlow power battery applications it was also easier to find a commerciallyavailable device that would operate from the nominal 2.4V batterysource.

An additional benefit of using the microcontroller according toexemplary embodiments of the present invention includes animplementation of a more sophisticated charging technique, according toyet another exemplary embodiment of the present invention, than istypically employed in traditional emergency lighting units. According toexemplary implementations of the present invention, a microcontrollercan provide timing periods function and monitoring of battery chargevoltage and current. Using these capabilities the design of the batterycharger according to exemplary embodiments of the present invention canprovide a quick bulk charge mode to quickly recharge the battery after adischarge while also providing a maintenance charge that significantlylowered (ten times less) maintenance mode power requirements for thecharger. The lower maintenance charge mode also provides the additionalbenefit of extending battery life by only charging the battery when itis required reducing the amount of wear on the battery.

FIGS. 1 through 5F illustrate exemplary implementations of circuitcomponents forming at least a portion of LED-based emergency lightingsystem or equipment according to an exemplary embodiment of the presentinvention as follows.

FIG. 1 is an illustrative example of a primary side flyback convertercircuitry according to an exemplary embodiment of the present invention,including AC-DC conversion and primary and auxiliary flyback circuits100, two level brown-out detection circuit 102, and flyback controllercircuits 104.

FIG. 2 is an illustrative example of a LED lamp circuit (one for eachlamp provided) 200 according to an exemplary embodiment of the presentinvention. According to an exemplary implementation, “NUD4700” component(which includes elements 206 and 208) and “MP4690” component (whichincludes elements 202 and 204) are not both used to build the lamp. Onlyone of the two devices needs to be populated to provide open circuitfailure protection for the series connected LEDs, such as “MX6LED” 210.

FIG. 3 is an illustrative example of a unit micro-controller 300according to an exemplary embodiment of the present invention.

FIG. 4a is an illustrative example of charger control and monitoringcircuits 400 according to an exemplary embodiment of the presentinvention associated with circuitry illustrated in FIG. 1 as shown bythe block diagram.

FIG. 4B is an illustrative example of DC-DC drive and LED statusindicators according to an exemplary embodiment of the presentinvention, including DC-DC gate drive circuits 402 and status LEDcircuits 404. FIG. 4C is an illustrative example of power and referencevoltage circuits according to an exemplary embodiment of the presentinvention, including Vbatt+ and bias output “OR” circuits 406, 3.3V LDOregulator circuit 408, bias output and power enable diode “OR” circuit410, and I charge reference filter circuit 412.

FIG. 5A is an illustrative example of DC-DC conversion and monitoringcircuits 500 according to an exemplary embodiment of the presentinvention. FIG. 5B is an illustrative example of charger voltage andcurrent control circuits 502 according to an exemplary embodiment of thepresent invention associated with circuitry illustrated in FIGS. 1 and4A as shown by the block diagram.

FIG. 5C is an illustrative example of DC-DC drive and 3.3V regulatorcircuits according to an exemplary embodiment of the present invention,including DC-DC drive circuits 504 and 3.3V regulator filter circuit506. FIG. 5D is an illustrative example of reset, device programming andtest switch circuits according to an exemplary embodiment of the presentinvention, including reset and device programming circuits 508 and unittest switch circuit 510. FIG. 5E is an illustrative example of a chargerreference voltage circuit 512 according to an exemplary embodiment ofthe present invention. FIG. 5F is an illustrative example of a chargerreference current circuit 514 according to an exemplary embodiment ofthe present invention

FIGS. 6 through 15 illustrate through the use of system state machinediagrams and flow charts exemplary implementations of LED-basedemergency lighting methodology or processes according to exemplaryembodiments of the present invention as follows.

FIG. 6 is an illustrative example of a unit system state machine whichincludes process flow and functionality as defined by the annotatedstates S600 through S610. FIG. 7 is an illustrative example of anotherunit system state machine which includes process flow and functionalityas defined by the annotated states S700 through S714, where states S700,S702, S704, S710, S712, S714 parallel states S600, S602, S604, S606,S608, S610, respectively, and further provide for a self-diagnosticscapability as shown by states S706, S708.

According to exemplary embodiments of the present invention, severalcharging techniques can be implemented including, but not limited to:(1) based on fixed bulk charge and trickle charge time interval todetermine an end-of-charge status, and (2) based on delta V/delta t<=0and a safety timer to determine an end-of-charge.

Referring to the state diagram of FIG. 8, in an exemplaryimplementation, charge method (1) is executed as follows:

The charger algorithm starts (S800) from Initialize state (S802) duringwhich it disables the charger, resets the battery status to not fullycharged, and resets all the time in fault condition counters. Then itwaits, for example 2 seconds, for stabilization and shifts to Idle state(S804).

In Idle state, it first checks charge current and shifts to ChargerFault state (S806) if the charge current is greater than a currentthreshold value (for example, 6 mA) for over, for example, a second. InCharger Fault state, it will wait, for example 5 seconds, and shiftsback to Initialize state (S802) and start over. If the charge current isless than or equal to the current threshold value, then it samples thecharge current amplifier output offset and stores it for futurecalculation. Then it shifts to Check Cell state (S808).

In Check Cell state, it samples the battery voltage. If the batteryvoltage is higher than voltage threshold value (for example, 3.3V for2-cell configuration or 6.6V for 4-cell configuration) for over, forexample, a second, then it shifts to Battery Fault state (S810). InBattery Fault state, it will wait, for example 5 seconds, and shiftsback to Initialize state and start over. If the battery voltage is nottoo high (for example, less than or equal to the voltage thresholdvalue), then it shifts to Bulk Charge state (S812).

In Bulk Charge state, it sets the charge current to a bulk charge value(for example, 200 mA) and enables the charger. Then it checks thebattery voltage. If the battery voltage is lower than a low voltagethreshold value (for example 1.8V) for, for example, over 10 ms, then itdisables the charger and shifts to Battery Fault state (S810). InBattery Fault state, it will wait 5 seconds and shifts back toInitialize state and start over.

Then, continuing in Bulk Charge state, it checks both the batteryvoltage and charge current. If the battery voltage is higher than thevoltage threshold value and there is no charge current (for example, thecharge current is less than 6 mA) for over, for example, a second, thenit is considered that there is no battery connected. So it shifts to NoBattery state (S814). In No Battery state, it will wait, for example 5seconds, and shift back to Initialize state (S802) and start over.

If there is a battery connected, then it checks if the battery voltageis too high as an indication of battery determination. If the batteryvoltage is higher than voltage threshold value (for example, 3.3V for2-cell configuration or 6.6V for 4-cell configuration) and the chargecurrent is greater than a current threshold value (for example, 6 mA)for over, for example, a second, then it shifts to Battery Fault state.In Battery Fault state, it will wait 5 seconds and shifts back toInitialize state and start over.

Continuing further in Bulk Charge state, if the battery voltage iswithin the normal range, then it checks charge current. If the chargecurrent is outside+/−10% of 200 mA for over a second, then it shifts toCharger Fault state. In Charger Fault state, it will wait 5 seconds andshifts back to Initialize state and start over.

If the battery voltage and the charge current are within the normalrange, then it checks if the bulk charge time is over, for example, 10hours. It stays in Bulk Charge state and check the above-mentionedbattery voltage, charge current and charge time parameters, for exampleevery 1 ms, until the bulk charge time is expired. It then shifts toTrickle Charge Low state (S816).

In Trickle Charge Low state, it first disables the charger. Then itchecks the battery voltage. If the battery voltage is lower than, forexample, a trickle charge threshold value (for example, 2.4V for 2-cellconfiguration or 4.8V for 4-cell configuration) for over a second, thenit shifts to Battery Fault state. In Battery Fault state, it will wait 5seconds and shifts back to Initialize state and start over.

According to an exemplary embodiment, trickle charge threshold value canbe proportional to a number of cells of the rechargeable battery, suchthat, for example, the trickle charge threshold value can be calculatedto be about 1.2V multiplied by N, where N is equal to a number of cellsof the rechargeable battery.

If the battery voltage is higher than voltage threshold value (forexample, 3.3V for 2-cell configuration or 6.6V for 4-cell configuration)for over, for example, a second, then it shifts to Battery Fault state.In Battery Fault state, it will wait 5 seconds and shifts back toInitialize state and start over.

According to an exemplary embodiment, voltage threshold value can beproportional to a number of cells of the rechargeable battery, suchthat, for example, the threshold value can be calculated to be about1.65V multiplied by N, where N is equal to a number of cells of therechargeable battery.

Continuing in Trickle Charge Low state, it checks the charge current. Ifthe charge current is greater than a current threshold value (forexample, 6 mA) for over, for example, a second, then it shifts toCharger Fault state. In Charger Fault state, it will wait 5 seconds andshifts back to Initialize state and start over. If the charge current isless than or equal to the current threshold value, then it checks if thetrickle charge low time is over 6 hours. It stays in Trickle Charge Lowstate and check the above-mentioned parameters every 1 ms until thetrickle charge low time is expired. It then shifts to Trickle ChargeHigh state (S818).

In Trickle Charge High state, essentially the same functions as in BulkCharge state are performed, except that the trickle charge highexpiration time is 30 minutes. After it expires, it shifts to TrickleCharge Low state.

If there is an emergency event during which the AC is lost, the systemstate machine will shift to emergency state and when the AC is restored,the state machine will reset the battery charger state machine and thecharger starts from Initialize state again.

In an exemplary implementation, charge method (2) is executed in asimilar manner to charge method (1). The main differences are asfollows.

In Bulk Charge state, it starts to check delta V/delta t after the bulkcharge starts for over 5 minutes. It checks the battery voltageincrements and stores the peak and calculates delta V/delta tcontinuously. If a delta V/delta t<=0 condition is met for a continuous90 minutes, then it shifts to Trickle Charge Low state. A secondarymethod of determination of end-of-charge is realized by a safety timerset for 12 hours. If the previously described delta V/delta t<=0condition is never met for over 12 hours, then it also shifts to TrickleCharge Low state.

The other difference is that, during Trickle Charge High state, thecharge current is set to 100 mA, and the duration of Trickle Charge Highstate is 1 minutes. The duration of Trickle Charge Low state is 5minutes.

FIG. 9 is an illustrative example of LED driver substrate machine whichincludes process flow and functionality as defined by the annotatedstates S900 through S910. FIG. 10 is an illustrative example of aninitialization state which includes process flow and functionality asdefined by the annotated states S1000 through S1012. FIGS. 11A and 11Bprovide an illustrative example of a battery charger state whichincludes process flow and functionality as defined by the annotatedstates S1100 through S1134.

FIG. 12 is an illustrative example of an emergency state which includesprocess flow and functionality as defined by the annotated states S1200through S1218. FIG. 13 is another illustrative example of an emergencystate which includes process flow and functionality as defined by theannotated states S1300 through S1326, where states S1300, S1302, S1304,S1310, S1312, S1314, S1316, S1322, S1324, S1326 parallel states S1200,S1202, S1204, S1206, S1212, S1214, S1216, S1208, S1210, S1218,respectively, and further provide for a self-diagnostics capability asshown by states S1306, 1308, 1318, 1320.

FIG. 14 is an illustrative example of test state (see for example, Teststate S708 of FIG. 7) which includes process flow and functionality asdefined by the annotated states S1400 through S1408. FIG. 15 is anillustrative example of a load-learn state (see for example, Load-learnstate S706 of FIG. 7) which includes process flow and functionality asdefined by the annotated states S1500 through S1508.

FIG. 16 provides an illustrative example 1600 of a battery voltage 1602and current charge 1604 profile for deltaV/deltaT method according to anexemplary embodiment of the present invention. As illustrated in FIG.16, according to an exemplary implementation, an amount of charge duringa given charging period can be set based on battery capacity C. Forexample, a bulk charge period can be about 200 mA which is a C/6.5charge rate for a 1300 mAh battery. On the other hand, the tricklecharge rate can have a lower value of about 100 mA during Tickle ChargeHigh state which is C/13 rate for a 1300 mAh battery. According to yetanother exemplary implementation on and off times of the charger duringthe trickle charge state are about one minute at C/13 during TrickleCharge High state followed by being about zero for about 5 minutes(Trickle Charge Low state).

According to yet another exemplary embodiment of the present invention,because the LED light source allowed the battery size to besignificantly reduced the design of the unit was able to incorporate anAC-DC switch-mode fly-back converter to replace the bulky 60 Hz magneticstep down transformer typically found in traditional emergency lightingunits. Moving to this type of circuit topology allowed a significantreduction in the size and weight of the charger and allowed the overallunit to assume a smaller size that is preferred by our customers. Inaddition to the reduction in size the fly-back converter style chargeris designed to operate from a wide supply range. Because the charger canoperate properly from 102-305 VAC, only two AC input supply wires arerequired. Traditional emergency lighting units with 60 Hz. magnetictransformers rely on voltage taps on the transformer primary to allowvoltages of 120V or 277V to operate the unit. Because the installer hasto select the proper voltage tap when the traditional emergency lightingunit is installed there is a chance that the wrong tap can be selected.When this happens in the field either the unit will not charge thebattery or damage to the transformer can occur. By moving to the widesupply range type fly-back converter the risk that the voltage tap canbe improperly connected is eliminated.

While the fly-back converter design eliminates the chance that the unitwill be improperly wired to the incorrect voltage tap, it does presentchallenges to properly detecting brownout voltage conditions. Priortechniques, such as one described in U.S. Pat. No. 7,256,556, addressedthis problem by integrating a microprocessor on the primary side of thefly-back converter to determine the connected input voltage and set abrownout threshold based on the input voltage level detected. While thissystem can detect all common distribution voltages between 102-305 VACand works independently of the connected line frequency (50 or 60 Hz)for voltages such as 220V 50 Hz, it is a very costly method ofestablishing the appropriate brownout threshold due primarily to thecost of the microprocessor on the primary side of the circuit. Giventhat the common distributor stock products only operate from 120 or 277V 60 Hz an exemplary embodiment of the present invention provides a lowcost method of generating a brownout signal at the appropriate voltagelevel without having to measure the incoming AC signal level todetermine the required brownout threshold level. By limiting theallowable input voltage to 120 or 277 VAC 60 Hz this invention realizesa low cost brownout detection circuit that does not require theemergency lighting unit to learn the connected AC voltage level when theunit is connected to the AC line.

According to an exemplary implementation of the present invention, theoperation of the unit is restricted to two valid input voltage rangesand one valid input operating frequency, whereby the brownout voltagelevels can be established with discreet circuitry prior to connection toan AC line voltage. Yet another exemplary implementation of the presentinvention uses a brownout detection pin provided on most AC-DC fly-backcontroller ICs. Using this pin eliminates the need to communicate thebrownout signal to the emergency lighting control and battery chargingcircuitry that is typically located on the low voltage isolatedsecondary side of the fly-back converter circuitry. Because the need tocommunicate the brownout signal to the secondary side of the circuit iseliminated in the invention the need to provide an isolation elementsuch as an opto-coupler to communicate the brownout signal across theisolation barrier is eliminated.

In an exemplary embodiment of the present invention, a brown-outdetection method is implemented as follows.

The unit is required to respond to two different brown-out levelsautomatically. When the input voltage is 120V nominal, it needs todetect brown-out at about 75% of 120V, which is about 90 VAC. When theinput voltage is 277V nominal, it needs to detect brown-out at about 75%of 277V, which is about 208 VAC. Additionally, at least about +/−2% ofhysteresis needs to be provided to prevent from possible oscillation atthe boundary condition.

In an exemplary implementation as illustrated in FIG. 1, VIPER17 ofsection 104 along with TS4431 of section 102 and a few peripheralcomponents are used to realize this 2-level brown-out detection method.Referencing the schematics of FIG. 1, pin 5 of VIPER17 is set for abrown-out threshold at 0.45V with 50 mV hysteresis. R1, R2, R7, and R10are used to sense AC input voltage and the threshold is set to be around150 VAC. So when the input voltage is lower than 150 VAC, R3, R6, R11,and R14 are used as a voltage divider to realize the first levelbrown-out detection function. When the input voltage is higher than 150VAC, TS4431 is engaged and puts R12 in parallel with R14 by its internalopen-collector transistor, so that R3, R6, R11, R14 and together withR12 are used as a voltage divider to realize the second level brown-outdetection function.

An exemplary operation of brow-out circuitry for an LED-based emergencylighting system or equipment according to an exemplary embodiment of thepresent invention is described as follows with reference to FIGS. 1through 5F.

Referring to the example of FIG. 1, the brown-out circuit shown insection 102 uses an open collector shunt reference (U2) in conjunctionwith a voltage divider circuit (R1, R2, R7, and R10) that creates a DCvoltage at pin 4 of (U2) that is proportional to the bulk DC inputvoltage created in section 100 of the circuit diagram by full waverectifying the incoming AC input voltage. This incoming DC referencesignal at pin 4 of (U2) that is proportional to the AC line is used toestablish a threshold where the open collector transistor in (U2) turnson or off. In this exemplary embodiment of the invention, that thresholdis set to approximately 150 VAC. This level was chosen to be above anyexpected high line conditions for a 120V nominal input voltage and wellbelow any expected low line condition for a nominal 277V input. Thismethod of predetermining an above or below threshold level inconjunction with the open collector switch in U2 gives the circuitry themeans to adjust the proportionality of the brownout signal used to turnon or off the fly-back control circuit IC via pin 5 on (U1) shown insection 104 of the circuit diagram.

In normal operation, according to an exemplary implementation, theproportionality of the voltage divider made up of (R1, R2, R7, and R10)is chosen such that an input to the reference pin of (U2) is less thanthe internal reference in the IC of 1.224V. Since the reference signalis lower than the internal reference signal, the output of the erroramplifier in (U2) controls the internal open collector transistor to theoff or open state. Because the open collector transistor in (U2) is offfor voltage levels less than approximately 150 VAC, the brown-outthreshold for the circuit is established by the voltage divider made upof (R3, R6, R11 and R14) and the brown-out detect pin threshold voltagelevel for the fly-back control circuit IC (U1). Without the opencollector switch in U2 switched off, the voltage divider made up of (R3,R6, R11 and R14) establishes a DC voltage at pin 5 of (U1) that goesbelow the internal brownout threshold level (0.45V) for pin 5 of (U1)when the AC input line voltage drops below approximately 80 VAC. Oncethis level is reached the fly-back control IC (U1) of the circuit turnsoff the fly-back converter and simulates a total power failure conditionon the output of the fly-back converter that cause the unit to move toemergency mode and operate the LED lamps.

In an exemplary implementation, (U1) is equipped with internalhysteresis circuitry such that the DC input level at pin 5 of (U1) hasto increase to (0.05V) for U1 to turn back on the fly-back converter sothe unit can enter the charging state again. The DC input level of(0.05V) at pin 5 equates to approximately 96 VAC input line voltagecondition. According to an exemplary embodiment of the presentinvention, such hysteresis circuitry can prevent a rapidly changingcircuit state condition if the incoming line was fluctuating between twopoints slightly above and below the AC line input voltage threshold.

Next, according to an exemplary implementation of embodiments of thepresent invention, for voltage levels above the 150 VAC threshold of thebrownout circuit shown in section 102 of the schematic of FIG. 1. The DCinput circuit level presented to pin 4 of (U2) is above the 1.224Vinternal voltage reference causing the internal error amplifier in (U2)to control on or close the switch connecting resistor R12 to ground.When the switch is closed (R12) is placed in parallel with (R14) in thevoltage divider circuit made up of (R3, R6, R11 and R14). By placing(R12) in parallel with R14 the value of the lower resistor in thedivider circuit is changed, in an exemplary implementation, from 13.7 kohms to 5.15 k ohms. Lowering the value of the lower resistor in thedivider circuit has the effect of requiring a higher AC input voltagelevel needed to generate the (0.45 VDC) level at pin 5 of (U1) requiredfor the unit to enter the brown-out state. The parallel combination ofresistors (R12 and R14) changes the proportionality ratio of the voltagediver such that the AC line voltage drops to approximately 220 VAC forthe unit to enter the brown-out mode. Likewise, according to anexemplary implementation, the new proportionality established by theresistors (R12, R14 parallel combination) establishes a threshold ofapproximately 230 VAC to turn off the LED lamps and move back to thecharging state.

Although exemplary embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions, and substitutions arepossible, without departing from the scope of the present invention.Therefore, the present invention is not limited to the above-describedembodiments, but is defined by the following claims, along with theirfull scope of equivalents.

We claim:
 1. A method for charging a rechargeable battery by a chargecurrent comprising the steps of (a) sampling a voltage of saidrechargeable battery and said charge current; (b) if said voltage ofsaid rechargeable battery is less than or equal to a voltage thresholdvalue and said charge current is less than or equal to a currentthreshold value, setting said charge current to a bulk charge value andapplying said charge current to said rechargeable battery for a bulkcharge time period; (c) if said voltage of said rechargeable battery isgreater than or equal to a low voltage threshold value and less than orequal to said voltage threshold value and said charge current is withina certain range of said bulk charge value, and a duration of said bulkcharge time period is greater than or equal to a bulk charge timeexpiration period, setting said charge current to a trickle charge lowvalue and applying said charge current to said rechargeable battery fora trickle charge low time period; (d) if said charge current is lessthan or equal to a current threshold value, and a duration of saidtrickle charge low time period is greater than or equal to a tricklecharge low time expiration period, setting said charge current to atrickle charge high value and applying said charge current to saidrechargeable battery for a trickle charge high time period; (e) if saidvoltage of said rechargeable battery is greater than or equal to saidlow voltage threshold value and less than or equal to said voltagethreshold value and said charge current is within a certain range ofsaid trickle charge high value, and a duration of said trickle chargehigh time period is greater than or equal to a trickle charge high timeexpiration period, setting said charge current to a trickle charge lowvalue and applying said charge current to said rechargeable battery; and(f) repeating said steps (a) through (e) at least once.
 2. The method ofclaim 1, wherein said voltage threshold value is proportional to anumber of cells of said rechargeable battery.
 3. The method of claim 1,wherein said certain range of said bulk charge value is within about+/−10% of said bulk charge value.
 4. The method of claim 1, wherein saidcertain range of said trickle charge high value is within about +/−10%of said trickle charge high value.
 5. The method of claim 2, whereinsaid voltage threshold value is about 1.65V multiplied by N, where N isequal to a number of cells of said rechargeable battery.
 6. A method forcharging a rechargeable battery by a charge current comprising the stepsof (a) sampling a voltage of said rechargeable battery and said chargecurrent; (b) if said voltage of said rechargeable battery is less thanor equal to a voltage threshold value and said charge current is lessthan or equal to a current threshold value, setting said charge currentto a bulk charge value and applying said charge current to saidrechargeable battery for a bulk charge time period; (c) calculating adelta V/delta t value of said voltage, wherein V is said voltage of saidrechargeable battery, and t is time; (d) if said delta V/delta t valueis less than or equal to about zero for a first conditional time period,or if during a second conditional time period said delta V/delta t valueis not less than or equal to about zero for said first conditional timeperiod, setting said charge current to a trickle charge low value andapplying said charge current to said rechargeable battery for a tricklecharge low time period; (e) if said current of said charger circuit isless than or equal to said current threshold value, checking at tricklecharge low check time intervals a duration of said trickle charge lowstate, and if said duration of said trickle charge low state is greaterthan or equal to a trickle charge low time expiration period, settingsaid charge current to a trickle charge high value and applying saidcharge current to said rechargeable battery for a trickle charge hightime period; and (f) repeating said steps (a) through (e) at least once.